chore: record main as merged into epic/app-onboard

The previous sync landed as a squash, so its content arrived but its
ancestry did not. Git still computed the merge base at the pre-sync commit,
which made the epic PR render every change from main as part of the epic's
own diff, 261 files instead of the epic's actual work.

This merge carries both parents so the merge base moves to main's tip. The
resulting tree is byte-identical to the current branch, so no file changes.

Author: DaniAkash